A three-way catalyst using rare metals such as rhodium (Rh), platinum (Pt) and palladium (Pd) is known for purifying the exhaust gas of an engine.
The three way catalyst makes exhaust gas harmless by reducing nitrogen oxides (NOx) and oxidizing carbon monoxide (CO) and hydrocarbons (HC).
However, the three way catalyst is not activated sufficiently if the temperature of the exhaust gas does not rise to some extent, and it cannot then exhibit its real purification performance.
Exhaust gas contains many hydrocarbons (HC) when the engine starts.
Tokkai Hei 2-56247 published by the Japanese Patent Office in 1990, discloses a catalyst unit comprising a second layer of a three-way catalyst having a rare metal as its main component formed on a first layer of zeolite with HC adsorbing properties as its main component.
In this catalyst unit, the zeolite adsorbs HC at a low temperature, and the desorbed HC from the zeolite is oxidized by the three-way catalyst which is activated by the rise of exhaust gas temperature.
However, the oxygen concentration in the exhaust gas decreases when the HC desorbs from the zeolite.
Consequently, in such an environment where the oxygen concentration has decreased, the three-way catalyst cannot exhibit its full performance which it exhibits at the stoichiometric air-fuel ratio.
Tokkai Hei 5-59942 published in 1993, Tokkai Hei 6-74019, 6-142457 published in 1994, and Tokkai Hei 7-144119, 7-102957 published in 1995 by the Japanese Patent Office, disclose a method wherein HC at low temperature is adsorbed on a HC adsorbent, and desorbed after the three-way catalyst is activated.
However, according to this prior art device, it was necessary to control the activation time of the three-way catalyst, and necessary to instal special equipment such as a bypass passage for hot gas and a heat exchanger to warm up the three-way catalyst.
In addition, it was necessary to use a large amount of rare metals so that the three-way catalyst which oxidizes the HC desorbed from the HC adsorbent, continues to have a high oxidizing performance in the long term.